1. Field of the Invention
The present invention relates to a technology for manufacturing and assembling a lightning arrester.
2. Description of the Related Art
An electric power facility such as an electric power station or an electric power substation needs to have good aseismic capacity for maintaining safe power supply. To achieve such aseismic capacity, air-insulated lightning arresters are installed in an electric power facility. Currently, an air-insulated lightning arrester predominately includes a porcelain bushing insulator (hereinafter, “porcelain type lightning arrester”). However, instead of a porcelain bushing insulator, a polymer insulator can also be used in an air-insulated lightning arrester (hereinafter, “polymer type lightning arrester”). A polymer insulator includes a zinc oxide element around which silicon rubber is directly molded. A polymer type lightning arrester is smaller and lighter as compared to a porcelain type lightning arrester, as well as has better aseismic capacity and pressure-relief capacity. For that reason, in recent years, there is an increasing trend of installing polymer type lightning arresters in an electric power facility.
A polymer type lightning arrester includes a plurality of nonlinear voltage resistors (i.e., zinc oxide elements) stacked to form a serially-connected resistor unit. The serially-connected resistor unit is sandwiched between two electrodes. The polymer type lightning arrester is covered from outside by integrally molding an outer insulating cover such as silicon rubber. However, a polymer resin such as silicon rubber cannot support on its own the structure of the polymer type lightning arrester. Thus, to increase the mechanical strength of the polymer type lightning arrester, insulating rods made of fiber-reinforced plastics are fixed between the electrodes.
Meanwhile, if an abnormal voltage such as a lightning induced surge that exceeds the capacity of a polymer type lightning arrester occurs, there is a possibility that the nonlinear voltage resistors get damaged. In that case, an internal electric arc occurs such that a large amount of gas is discharged because of the arc heat. That causes a sudden increase in the internal pressure of the polymer type lightning arrester. To prevent the polymer type lightning arrester from exploding due to sudden increase in the pressure, the outer insulating cover rips such that the gas is released outside such that the internal pressure decreases. Moreover, the insulating rods prevent the broken pieces of the constituent elements of the polymer type lightning arrester from flying around and to maintain the electrodes intact in their fixed positions.
Japanese Patent Application Laid-open No. 2002-270406 discloses a lightning arrester in which an insulating rod with a retaining member on one end is fixed to a pair of electrodes in the following manner. First, one end of the insulating member is inserted into an insulating-rod fixing hole created on one of the electrodes. The other end of the insulating rod is then inserted through a cylindrically protruded hole created on the other electrode. Subsequently, the part of the insulating rod protruding from the protruded hole is crimped. However, when such a structure is subjected to a stress that works in the direction of pushing away the electrodes, then a tensile force in the direction of unfastening the crimping is absorbed by the frictional force between the insulating rod and the electrode having the protruded hole. As a result, there is a possibility that the insulating rod gets unfixed from the pair of electrodes.
Generally, a polymer type lightning arrester is used in multiple stacks depending on a rated voltage. However, because the rated voltage of a single polymer type lightning arrester is far less than that of a single porcelain type lightning arrester, it is necessary to stack a large number of polymer type lightning arresters to obtain the same rated voltage as that of the porcelain type lightning arrester. As a result, the process of assembling the lightning arrester unit becomes complicated thereby affecting the productivity of the polymer type lightning arresters.
Moreover, while stacking a large number of polymer type lightning arresters, it is necessary to use an equivalent number of connecting components to interconnect the polymer type lightning arresters. Thus, the necessary number of components for assembly increases thereby affecting efficiency in the on-site installation of the lightning arrester unit.